Plasma arc torches are widely used in the cutting or marking of metallic materials. A plasma torch generally includes an electrode mounted therein, a nozzle with a central exit orifice mounted within a torch body, electrical connections, passages for cooling and arc control fluids, a swirl ring to control fluid flow patterns in the plasma chamber formed between the electrode and nozzle, and a power supply. The torch produces a plasma arc, which is a constricted ionized jet of a plasma gas with high temperature and high momentum. Gases used in the torch can be non-reactive (e.g. argon or nitrogen), or reactive (e.g. oxygen or air).
In operation, a pilot arc is first generated between the electrode (cathode) and the nozzle (anode). Generation of the pilot arc may be by means of a high frequency, high voltage signal coupled to a DC power supply and the torch or any of a variety of contact starting methods.
One known configuration of a plasma arc torch includes an electrode and a nozzle mounted in a special relationship relative to a shield. The nozzle is surrounded by the shield and aligned relative to a longitudinal axis extending through the nozzle and the shield such that the nozzle orifice and shield orifice are concentric relative to one another. A relatively small plasma gas flow passes through the torch and exits through the nozzle orifice. A relatively large shield gas flow passes through the space between the nozzle and the shield. The plasma gas flow passes through the nozzle exit orifice along the axis, while the shield gas flow passes through the gap at an angle relative to the axis. As such the shield flow impinges on the plasma gas flow. After impingement, the plasma arc and shield flows pass through the shield orifice together. This process can disrupt the plasma gas flow, encouraging shield gas entrainment which can result in a degraded cutting performance.
It is therefore the object of the present invention to provide an improved torch tip for a plasma arc torch, which provides a substantially columnar shield flow that does not substantially interfere with the plasma arc.